Added FILETIME conversions

Examples-and-Recipes.md

@@ -25,6 +25,7 @@ This page contains examples and recipes contributed by community members. Feel f
 - [What is the epoch difference between Unix Time and GPS time?](#unix_gps_epoch_diff)
 - [How to convert to/from C++ Builder's TDate and TDateTime](#TDate)
 - [How to convert to/from QDate](#QDate)
+- [How to convert to/from Windows' FILETIME](#FILETIME)
 
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@@ -1240,6 +1241,44 @@ Here are functions you can use to convert between [`QDate`](http://doc.qt.io/qt-
 
 These work by simply adjusting the epoch of these two types.
 
+<a name="FILETIME"></a>
+### How to convert between Windows' `FILETIME` and `system_clock`
+(by [Billy O'Neal](https://github.com/BillyONeal))
+
+Here are functions and typedefs you can use to work with Windows' [FILETIME](https://msdn.microsoft.com/en-us/library/windows/desktop/ms724284.aspx) structure.
+
+Assuming `system_clock` is based on the Unix epoch:
+
+    using std::ratio;
+    using std::chrono::duration;
+    using std::chrono::duration_cast;
+    using std::chrono::system_clock;
+
+    // filetime_duration has the same layout as FILETIME; 100ns intervals
+    using filetime_duration = duration<int64_t, ratio<1, 10'000'000>>;
+    // January 1, 1601 (NT epoch) - January 1, 1970 (Unix epoch):
+    constexpr duration<int64_t> nt_to_unix_epoch{INT64_C(-11644473600)};
+
+    system_clock::time_point FILETIME_to_system_clock(FILETIME fileTime) {
+        const filetime_duration asDuration{static_cast<int64_t>(
+            (static_cast<uint64_t>(fileTime.dwHighDateTime) << 32)
+                | fileTime.dwLowDateTime)};
+        const auto withUnixEpoch = asDuration + nt_to_unix_epoch;
+        return system_clock::time_point{
+            duration_cast<system_clock::duration>(withUnixEpoch)};
+    }
+
+    FILETIME system_clock_to_FILETIME(system_clock::time_point systemPoint) {
+        const auto asDuration = duration_cast<filetime_duration>(
+            systemPoint.time_since_epoch());
+        const auto withNtEpoch = asDuration - nt_to_unix_epoch;
+        const uint64_t rawCount = withNtEpoch.count();
+        FILETIME result;
+        result.dwLowDateTime = static_cast<DWORD>(rawCount); // discards upper bits
+        result.dwHighDateTime = static_cast<DWORD>(rawCount >> 32);
+        return result;
+    }
+
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